Hey! I am testing this mod now and it looks really cool! I'm planning on using this mod in my private modpack I'm making currently.
I do have few suggestions if you don't mind:

1. Add reverse crafting recipy for Dual Glass Pane (so you get two Glass Panes back) - I think this one is simple fix - just add 17 more recopies.
2. Alternative placement option: Placing Glass Pane on each side of the block separately - With that as an option, we could place Panels of different colors together, as well as in shapes we currently can't do in single block space (notably just an "U" shape, with single wall lacking).
3. Ability to place Iron Bars on Edge similarly to your Glass Panes - this one I don't think needs explanation.
4. [Optional] Ability to place your Glass Panes using the same Glass Pane the Vanilla Equivalent uses - A placement of Glass Pane would be determined by space where you are looking at on the block:
   Center of Wall of the Block - Vanilla Glass Pane
   Corner of two Walls of the Block - Edge Aligned Glass Pane
   Corner of three Walls of the Block - Corner Aligned Glass Pane (with two Glass Panes, one on each corner)
   Maybe to make it easier to use this mod would be to add visual representation on how the block would look like after placing while looking at the block and you are holding the glass pane.
5. [Optional] Add Support for mods related to Glass, Bars or blocks with similar model:

• Quark
  Glass blocks: Framed Glass, Dirty Glass panes, as well as Crystal Sheets (does it count as glass? They have model similar to Glass Panes...)
  Bar blocks: Golden Bars
  [https://www.curseforge.com/minecraft/mc-mods/quark]

• Glasscutter
  No new Panes, but this mod adds new crafting block dedicated to Cutting Glass. It does introduce few Glass Blocks you can obtain from this block: Glass Stairs, Glass Slabs... well that's it... but for all 17 variants of glass!
  Well compability there would be to add recipies for your Panes, but it's more on the other mod side so I will contact mod author of that mod as well.
  Since this mods focuses on Glass, there are no Bars to add from there.
  [https://www.curseforge.com/minecraft/mc-mods/glasscutter]

• Additional Bars
  This one focuses purely on Bars... and there is a lot of them:
  New Iron Bars, Golden Bars, a lot of Wooden Bars and even it have compability with Biomes of Plenty and introduced Wooden Bars made out of new types of trees.
  And since this mod focuses on Bars... no new Glass Panes.
  [https://www.curseforge.com/minecraft/mc-mods/additional-bars]

And that's mostly mods I'm working around now.

Surely there are more mods out there which add more glass panes, but because there would be always more and more mods which would add some sort of block with Bar or Pane model, I'm not expecting you to make them all. Honestly I'm not expecting you to make those three either - that's a time investment as well and surely there are many things you would want to develop with your mod first.

What you think about those suggestions? :D

Here are my initial thoughts...

1. Implemented in version 1.3.0 :)
2. That is a fairly big issue since PITG panes are not multipart. I'll have to take a look if this would be more work than the original mod lol. Where this really becomes a problem is on the corners where two panes of different color meet. What color should the top and bottom edges of the adjoining corner be? A lot of block state logic went into getting the adjoining edges to not fight each other when more than one pane are overlapping. I want to make sure that the resulting structures are aesthetically pleasing and consistent. There is still more work to be done in this mod to take care of hidden surface removal so there are no areas where looking through the pane at an angle shows parts of unintended textures. I can add a U shaped version that takes three panes to craft but it would be a single color.
3. I keep going back and forth on this. It would be easy to add it to PITG but I wonder if having one bar item in a mod all about glass panes would confuse people. I may consider making a new mod that provides edge aligned bars.
4. This probably won't happen. I have been staying away from changing any vanilla block behavior for a number of reasons.
5. Integration to other mods is on the ToDo list. The first mod I want to support is Chisel. I'll have to take a look at Quark and Glasscutter. Since PITG has its own blocks and doesn't alter any other block behavior, integration with Additional Bars is less likely. You may have better luck talking to the author of that mod.

PITG version 1.3.0 has been released with the reverse dual pane recipes

Big thanks for answer!
Also... I suggested Additional Bars mostly as a followup for the suggestion to add Bars in general to your mod. I understand if you don't want to mix up the two...

If you decide to make separate mod for Bars... would you consider adding Edge Aligned Bars from Additional Bars mod?